We succeeded in developing wavelength conversion in quantum realm, which has been covered by many Japanese newspapers and Physics Today. We observed velocity slowing down in microtroid optical cavities. We have also investigated the tolerance, assuming our original quantum key distribution system, for practical "nearly single-photon source" and clarified that the tolerance is very practical, which is a very good news to the experimentalists. This research result was covered by Nihon Keizai Shinbun.